Licence Change

Hello all,
I have changed the project licence to GPLv3 now that Codeplex supports that licence.

This project was previously licensed under Apache 2.0, which I believed was compatible with GPLv3 (the licence that mysqtuner.pl is licensed under), but after further investigation, I found that it was not.

This change should fix any potential licensing issues between this project and the original mysqltuner.pl project.